1. Field of the Invention
The present invention relates to a matrix-type liquid crystal display device, and is more particularly associated with technology relating to the viewing angle characteristics of this liquid crystal display device.
2. Description of the Related Art
A matrix-type display device such as a liquid crystal display device usually comprises two substrates: a thin film transistor array substrate (hereinafter “TFT array substrate”) whereon thin film transistors (hereinafter “TFTs”) and so forth are established, and an opposite substrate whereon a color filter, black matrix, and so forth are established, with a display material such as liquid crystal held between those substrates. Image display is realized by selectively applying voltage to this display material.
FIG. 17 shows an equivalent circuit for a TFT array substrate. As shown in FIG. 17, the pixels are arranged in a matrix. In FIG. 17, G1, G2, and G3 are scanning signal lines (hereinafter “gate lines”). S1, S2, and S3 are picture signal lines (hereinafter “source lines”) Cs1, Cs2, and Cs3 are storage capacitive electrode lines (hereinafter “Cs lines”) for stabilizing the potential of the pixel electrodes during the holding period. 111 through 133 are TFTs. These TFTs function as switching elements and control the charging and discharging of charge to the pixel electrodes. 211 through 233 are storage capacitors (hereinafter “Cs capacitors”) and are created by forming an insulating film between the pixel electrodes and Cs lines. The pixel electrodes are formed with transparent electrodes such as ITO (Indium tin oxide) 311 through 333 are liquid crystal capacitors Clc formed between the opposite substrate and liquid crystal.
Turning the TFT ON and OFF is controlled by applying voltage pulses to the gate lines connected to the gate electrodes of the TFTs. The pixel electrodes are connected with the source lines through the TFTs. The amount of charge in the pixel electrodes varies according to the signal level in the source lines and the potential of the pixel electrodes is established. The displacement amount of the liquid crystal changes according to the voltage across the pixel electrodes and opposite electrodes and changes the quantity of light transmitted from the back surface. Consequently, the optical signal change is controlled and the image is displayed by controlling the signal level of the source lines.
In order to improve image quality, it is necessary to reduce as much as possible the variation of pixel potential due to the change in signal level of the gate lines and so forth. For this reason, the Cs capacitors 211 through 233 are established on the pixel electrodes and provide a large total capacitance for the pixel. The Cs capacitors are formed by establishing an insulating film between the Cs lines, at the same potential as the opposite electrodes, and the pixel electrodes.
Next, FIG. 18 shows the pixel layout in a conventional TFT array substrate. FIG. 19 shows a cross sectional view of the pixel in FIG. 18 at the line A–A′.
In FIG. 18, 2 shows a gate line, 4 shows a semiconductor thin film, 7 shows a source line, 8 shows a source electrode, 9 shows a drain electrode, 11 shows a Cs line, and 14 shows a pixel electrode. In FIG. 19, 1 shows the glass substrate, 2 shows the TFT gate line, 3 shows the gate insulating film, 4 shows the semiconductor thin film, 8 shows the source electrode, 9 shows the drain electrode, 11 shows the Cs line, 14 shows the pixel electrode, and 103 shows the insulating film.
The manufacturing method for a conventional TFT array substrate is explained using FIG. 19. First, a metal film to become the gate electrode is formed on the glass substrate 1, a resist pattern is developed, and the gate electrode 2 is formed. After the resist is removed, as shown in FIG. 19, the gate insulating film 3 and the semiconductor thin film 4, comprising an i layer and n layer, are formed from the bottom, a resist pattern is developed, and the semiconductor thin film 4 is etched. After the resist is removed, an ITO thin film, to become the pixel electrode, is formed, the resist is developed, and the ITO film is etched, whereby the pixel electrode 14 is formed. After the resist is removed, a metal film is formed, a resist pattern is developed, and the source electrode 8 and drain electrode 9 are formed by etching. Furthermore, all of the n layer and part of the i layer on the TFT back channel side are etched (back channel etching) and the insulating film 103 is formed.
The manufacture and functions of a conventional TFT are explained next using an example. In the case of charging charge to the pixel electrode 14 in the TFT shown in FIG. 18, positive voltage of about 9 V is applied to the source electrode 8 and positive voltage of around 20 V is applied to the gate electrode 2. The TFT is thereby turned on and the drain electrode 9 and pixel electrode 14 are charged to about 9 V. Thereafter, when the potential of the pixel electrode increases sufficiently, negative voltage of about −5 V is applied to the gate electrode 2, the TFT is turned off, and the charge is confined in the pixel.
However, in addition to the TN (twisted nematic) type, various operating modes are used for the operating modes of a liquid crystal display element, for example, the IPS (in plane switching) type, VA (vertical alignment) type, and so forth for widening the viewing angle. FIG. 20 shows the relationship (V-T characteristics) of the voltage applied to the liquid crystal and the transmittance in the normally white (black display on a white background) mode for a TN-type liquid crystal display element. As shown in FIG. 20, there is a difference of about 1 to 2 V, for example, between the voltage at the start of the change in transmittance (threshold voltage Vth) and the voltage when the transmittance change has almost ended (saturated voltage Vsat). In an active matrix-type liquid crystal display element using a TFT as a switching element, a tonal display is made by establishing a number of voltage levels between this Vth and Vsat.
In a TN-type liquid crystal display element, there is a large change in transmittance when changing viewing angle based on the operating principles, and the narrow range of the viewing angle is a problem. The conventional art for improving this problem is discussed in detail next using the drawings.
A proposed technology is to establish a range where the electrical field intensity applied to the liquid crystal varies while the same voltage is applied across the pixel electrode and the opposite electrode, opposite thereto and holding the liquid crystal therebetween, in one pixel. In this conventional art, as shown in FIG. 19, the insulating film 103 is formed on the upper layer of the pixel electrode 14, and then a region A where the insulating film 103 is removed and a region B where the film remains are formed on the pixel electrode 14. Actually, because the alignment layer is formed on the TFT array substrate and the opposite substrate, an alignment layer is formed on the pixel region in FIG. 18. The voltage drop occurs in this portion of the alignment layer as well, but at a level that can be ignored, so there is no problem even if this alignment layer is omitted in the following explanation.
In the region A of the pixel electrode 14 from which the insulating film 103 is removed, voltage applied across the pixel electrode 14 and opposite electrode is applied to the liquid crystal. In the region B where the insulating film 103 remains, a voltage drop occurs in the portion with the insulating film and the voltage applied across the pixel electrode 14 and the opposite electrode is not applied without any change thereto. In other words, in the pixel electrode 14, the regions A and B, having mutually different characteristics of voltage applied across the electrodes to transmittance (V-T characteristics), are established within one pixel.
FIG. 20(a) shows the V-T characteristics curve of a pixel in the region A with the insulating film 103 removed. FIG. 20(b) shows the V-T characteristics curve occurring in the region B where the insulating film 103 remains. If each pixel is sufficiently small compared to the entire liquid crystal display device, the V-T characteristics curve is projected to an observer as shown in FIG. 20(c) which is the sum of the characteristics in FIG. 20(a) and (b). As a result, the difference between Vth and Vsat is substantially expanded.
With the expansion of the difference between Vth and Vsat, the narrowness of the viewing angle range during tonal display is also reduced. FIG. 21 shows the situation of the change of the V-T characteristics due to the viewing angle direction. FIG. 21(a) shows the case where there are no regions on the pixel electrode with different electrical field intensities applied. FIG. 21(b) shows the case where there is a region on the liquid crystal with part of the insulating film on the pixel electrode removed as shown in FIG. 18 and with different electric field strength applied.
In a liquid crystal display element with the TN-type operating mode, the relationship between the applied voltage and transmittance changes from the solid line to the dotted line because of changing the viewing angle direction. As a result, in the case where there are no regions on the pixel electrode with different electric field strength, the transmittance changes greatly with a change in the viewing angle direction as shown in FIG. 21(a). On the other hand, as shown in FIG. 21(b), in the case where there is a region with different electric field strength, the voltage shift width of the V-T characteristics, due to the change in the viewing angle direction when regions of different characteristics are seen as a single pixel, is about the same. As a result, the change in transmittance due to the viewing angle direction is small corresponding to the portion with a gentle change in transmittance due to the applied voltage and the viewing angle range becomes wide.
On the other hand, various methods are proposed as methods for expanding the viewing angle for liquid crystal display devices using the TN mode. For example, these include a method for expanding viewing angle by varying the array direction of the liquid crystal molecules in the pixels with a multidomain structure, in other words by varying the rising direction of the liquid crystal molecules when voltage is applied; and a method for expanding viewing angle by reducing the slope of the voltage-brightness characteristics by manufacturing the display so that different voltages are applied to the liquid crystal in a single pixel. Furthermore, there is a method for expanding the viewing angle as a result of suppressing the light leakage, in the black display state with voltage applied to the liquid crystal, by inserting an optical compensating film such as a retardation film between the liquid crystal and polarizing plate.
Gray-scale inversion in half tones gray is an issue for the viewing angle characteristics. In the TN mode, gray-scale inversions do not easily occur upwards and horizontally, but gray-scale inversions occur easily downwards. FIG. 22 shows the relationship between the vertical relative transmittance and angle in the case of applying voltage so that the relative transmittance is 100%, 75%, 50%, 25%, and black display at normal to the panel surface of a conventional TN mode liquid crystal display device. As clear from the drawing, when the half tone gray display is observed at about −20°, there is an intersection with the relative transmittance curves and it is understood that the display quality markedly deteriorates with the excessively dark image from downwards.
As discussed above, in order to expand the viewing angle range, it is necessary to create regions with different electric field strength applied to the liquid crystal in one pixel. In order to create such regions, it has been proposed to form a region with an insulating film removed and a region with that film remaining on the pixel electrode. In order to expand the viewing angle range, however, it is necessary to reduce the inclination of the characteristics curve of the total of a plurality of V-T characteristics and it is necessary to form a thick insulating film on and then to remove this thick insulating film from the same pixel electrode.
By employing this type of constitution, large steps are formed in the insulating film on the same pixel electrode. Consequently, even if an alignment layer is applied in the panel process, the steps cannot be filled in and large steps will be formed in the individual pixels of the liquid crystal display device. The side walls and bases of the steps become a cause of light leakage because they make it difficult to apply the alignment process by rubbing treatment or the like in the panel process and make it difficult for the liquid crystal to align in the desired direction even when voltage is applied. For this reason, a normally white type of liquid crystal display device has a structural problem such that, even if sufficient voltage is applied for the black display state, the brightness of the black state becomes high because of the light leakage and this brings about reduced contrast.
In recent years, the requirements for high contrast have been increasing more and more with the employment of liquid crystal display devices in television set displays. On the other hand, there are also problems of gray-scale inversion occurring in half tone grays.